The invention discloses a graded pyrolysis gasifying system and application thereof, and a treatment method of leather-producing wastes. The system comprises a first rotary kiln and a second rotary kiln, wherein the first rotary kiln comprises a leather crushing feeding opening, a high-temperature smoke inlet, a medium-temperature smoke outlet, a first rough gas outlet and a second byproduct discharging opening; the second rotary kiln comprises a leather sludge feeding opening, a medium-temperature smoke outlet, a first rough gas outlet and a sludge slag discharging opening; the medium-temperature smoke outlet of the first rotary kiln is communicated with the medium-temperature smoke inlet of the second rotary kiln; the high-temperature smoke inlet is communicated with a secondary combustion chamber; the first rough gas outlet and/ or the second rough gas outlet is communicated with the secondary combustion chamber. The treatment method of the leather-producing wastes is clean and efficient; the leather-producing wastes can be reduced by 90% or more.